Display device and electronic device

ABSTRACT

According to the present invention, as a current-driven light-emitting means is driven by a current-adding D/A converter driving means, it is possible to drive the light-emitting means with large driving power. Moreover, electric power consumption may be reduced by the generation of wasteful drive currents being suppressed. Therefore, obtained are images of high intensity in an efficient manner with low electric power consumption.

TECHNICAL FIELD

The present invention relates to the technical field of a display devicecomprising a light-emitting element per pixel and an electronicapparatus comprising such display device.

BACKGROUND ART

Conventionally, with display devices comprising a light-emitting elementper pixel, after a digitized image signal is converted into an analogimage signal, such analog image signal is applied to the light-emittingelements.

Here, in order to convert the digital image signal into an analog imagesignal, it is necessary to utilize a so-called digital/analog converter(hereinafter referred to as a “D/A converter”).

As such D/A converter, so-called capacitive D/A converters and so-calledresistive D/A converters are known.

Among the above, as the resistive D/A converter, there is a D/Aconverter utilizing so-called ladder resistance wherein resistors areconnected in a ladder shape. As D/A converters utilizing ladderresistance may be integrated easily, they are preferable for beingincorporated into an active-matrix type display device.

Nevertheless, in order to realize large driving power upon using a D/Aconverter utilizing ladder resistance, it is necessary to lower thevalue of resistance of the respective resistors structuring the D/Aconverter. Thus, the overall electric power consumption will increase.Particularly in an active-matrix type display device, electric powerconsumption is severe since D/A converters are necessary per multitudeof light-emitting elements.

Moreover, in order to realize large driving power upon using acapacitive D/A converter, it is necessary to increase the capacitancevalue within the D/A converter. Thus, integration becomes difficult.

DISCLOSURE OF THE INVENTION

The present invention realizes a display device capable of drivinglight-emitting elements utilizing drive currents having a large currentvalue and lowering electric power consumption by suppressing currentconsumption, and an electronic apparatus employing such display device.

The display device of the present invention comprises:

a plurality of current-driven light-emitting means respectivelycontained in a plurality of pixels formed in a matrix shape; and

driving means for converting, by adding a current having a current valuecorresponding to the digital value contained in the digital data signal,the digital data signal into an analog data signal, applying the analogdata signal to the light-emitting means, and driving the light-emittingmeans.

The plurality of pixels is formed on a substrate, such as a transparentsubstrate. Thin-film current-driven light-emitting elements(light-emitting elements wherein the luminance degree changes inproportion to the current quantity to be flowed into the diode) may beused as the plurality of light-emitting means.

As the driving means generates an analog data signal by adding acurrent, the light-emitting means is driven with large driving power andelectric power consumption is lowered by the generation of wastefuldrive currents being suppressed.

Preferably, the D/A converter includes a current mirror circuit forgenerating a current having a current value corresponding to the digitalvalue contained in the digital data signal.

More preferably, the current-adding D/A converter includes a currentmirror circuit for generating a current having a current quantitycorresponding to the digital value contained in the digital data signal.By including a current mirror circuit, the analog data signal may besupplied to the light-emitting means efficiently.

Preferably, the driving means comprises additional-basic-currentapplication means for constantly applying a prescribed additional basiccurrent to the light-emitting means while making the light-emittingmeans illuminate in correspondence with the digital data signal.

The additional basic current may be a current having a prescribedcurrent quantity set in advance and lower than the minimum currentquantity within the range of currents in which the luminance of thelight-emitting polymer changes in proportion to the current-luminanceproperty of the light-emitting means.

By constantly applying the additional basic current to thelight-emitting means while the light-emitting means is illuminating, theluminance of the light-emitting means will be a luminance in proportionto the current quantity of the analog data signal. Thereby, obtained areimages accurately corresponding to the supplied digital data signal.

A display device of the present invention comprises:

a plurality of current-driven light-emitting means respectivelycontained in a plurality of pixels formed in a matrix shape;

data line driving means for converting, by adding a current having acurrent value corresponding to the digital value contained in thedigital data signal, the digital data signal into an analog data signal,applying the analog data signal to the light-emitting means, and drivingthe light-emitting means;

a scanning line for supplying a scanning signal;

a data line connected to the data line driving means and for supplyingthe analog data signal; and

switching means connected to the scanning line, the data line, and thelight-emitting means inside the pixel and for applying the analog datasignal to the light-emitting means in correspondence with the scanningsignal supplied from the scanning line and driving the light-emittingmeans.

As the data line driving means generates an analog data signal by addinga current, the light-emitting means is driven with a large driving powerand electric power consumption is lowered by the generation of wastefuldrive currents being suppressed.

By providing a switching means to each light-emitting means, realized isan active-matrix type display device capable of controlling the drive ofthe light-emitting means for each pixel. Thus, displayed are images ofhigh resolution.

Preferably, the switching means is formed of a thin-film transistor(hereinafter referred to as “TFT”); for example, apolysiliconthin-filmtransistor. By employing the polysilicon thin-filmtransistor, suppressed is the lowering of the driving power pursuant toa long-term flow of strong current.

Preferably, the data line driving means includes a current-adding D/Aconverter for converting, by adding a current having a current valuecorresponding to the digital value contained in the digital data signal,the digital data signal into an analog data signal.

More preferably, the D/A converter includes a current mirror circuit forgenerating a current having a current value corresponding to the digitalvalue contained in the digital data signal. By including a currentmirror circuit, the analog data signal may be supplied to thelight-emitting means efficiently.

The additional basic current may be a current having a prescribedcurrent quantity set in advance and lower than the minimum currentquantity within the range of currents in which the luminance of thelight-emitting polymer changes in proportion to the current-luminanceproperty of the light-emitting means.

By constantly applying the additional basic current to thelight-emitting means while the light-emitting means is illuminating, theluminance of the light-emitting means will be a luminance in proportionto the current quantity of the analog data signal. Thereby, obtained areimages accurately corresponding to the supplied digital data signal.

The light-emitting means of the present invention is preferably formedof a light-emitting polymer. By employing a light-emitting polymer,obtained are images of high intensity.

An electronic apparatus of the present invention comprises the displaydevice of the present invention. Thus, it is possible to display imageson the electronic apparatus of the present invention efficiently andwith low electric power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing the overall structure of the displaydevice according to the present invention.

FIG. 2 is a plan view showing the specific structure of the pixelportion of the display device according to the present invention.

FIG. 3 is an equivalent circuit of the pixel portion of the displaydevice according to the present invention.

FIG. 4 is a block diagram showing the structure of the (data line) drivecircuit.

FIG. 5 is a circuit diagram showing the detailed structure of the D/Aconverter according to the present invention.

FIG. 6 is a diagram showing the current-luminance property in thelight-emitting polymer.

FIG. 7 is a block diagram showing the schematic structure of theelectronic apparatus according to the present invention.

FIG. 8 is a front view showing the appearance of the personal computerof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION (I) Embodiment of the DisplayDevice

The best mode for carrying out the invention is now explained withreference to the drawings.

Foremost, explained with reference to FIG. 1 is the summary of theoverall structure of the active-matrix type display device employed inthe present invention.

As shown with the plan view thereof in FIG. 1, with the display device 1of the present embodiment, the center of the substrate; i.e.,transparent substrate 10, becomes the display 2 where images areactually displayed. Around the display 2 on the transparent substrate10, at the top and bottom of FIG. 1, a data line drive circuit 3 and aninspection circuit 4 are formed. The data line drive circuit works asthe drive circuit (or data line drive circuit) for outputting an imagesignal to the data line 6 based on the images to be displayed. Theinspection circuit 4 is used for inspecting the quality, defects, etc.of the display device 1 during manufacture or the shipment thereof.

Moreover, around the display 2 on the transparent substrate 10, at theleft and right sides of FIG. 1, a scanning line drive circuit 5 isformed. This scanning line drive circuit 5 outputs a scanning signal tothe scanning lines 7 based on the images to be displayed.

Furthermore, on the transparent substrate 10, mounting terminals 9 areformed on the exterior of the inspection circuit 4. The mountingterminals 9 are used for externally inputting an image signal, as wellas various voltage and pulse signals.

Here, pixels 11 are respectively formed in correspondence with theintersection of the data line 6 and the scanning line 7 inside thedisplay 2. Within a single pixel 11, as described later (cf. FIG. 3), alight-emitting polymer as the light-emitting means, a TFT as the drivingmeans, and so on are formed.

In addition, capacity lines 8 for accumulative capacity describe later(cf. FIG. 2) are arranged on the display 2. The capacity lines 8 areparallel to the scanning lines 7 within the respective pixels 11.

Next, explained is the structural members contained in the pixel 11 withreference to FIGS. 2 and 3. FIG. 2 is a plan view showing thearrangement of the TFT and so on formed inside the pixel 11 withthin-film technology. FIG. 3 is an equivalent circuit regarding a singlepixel 11.

As shown in FIG. 2, a pixel electrode 12 and TFT 13 are formed inside asingle pixel 11. The pixel electrode 12 is used for applying current tothe light-emitting polymer as described later. The TFT 13 works as aswitching means for supplying an image signal from the data line 6 tothe pixel electrode 12. The pixel electrode 12 and the TFT 13 are formedas thin films. The TFT 13 further comprises a semiconductor layer(semiconductor layer having a channel region, source region, and drainregion) formed from polysilicon.

A capacity line 8 is arranged in the position opposite to the pixelelectrode 12. The line 8 forms the accumulative capacity described later(cf. FIG. 3) together with the pixel electrode 12.

Here, the light-emitting polymer is described in detail.

The light-emitting polymer is formed as a thin film in the displaydevice 1 of the present embodiment. Specifically, the light-emittingpolymer is formed by laminating a spacer layer, organic luminescentlayer, and hole injection layer, and it self-illuminates in proportionto the current quantity of the flowing current.

The light-emitting polymer is a light-emitting element in which theilluminant contributing to the illumination is an organic material. Theprinciple characteristics thereof are as follows.

(1) It can be easily made into ink and solution, and has a highpotential for being formed into a thin film. Thus, it can be made into athin film in a short period of time, and it is also easy to make amulti-layered thin film.

(2) The physical strength is strong upon being formed into a thin film.Thus, crystallization and cohesion due to aging are difficult to occur.Moreover, display defects such as sunspots will not be generated easily.

(3) Patterning into a desired shape is easy. It is also possible to usematerials having photosensitivity. Thus, it is possible to conductpatterning directly with inkjet technology, printing technology, or thelike.

(4) The molecular design is extremely diverse, and it is easy to addfunctions or control luminescent colors.

As such organic materials, specifically, the following substances may beused. As those having a luminescent color from red to orange, forexample, poly [2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene](so-called MEH-PPV), poly[2-(3,7-dimethyloctyloxy)-5-methoxy-1,4-phenylenevinylene] (so-calledOC₁C₁₀PPV) or poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylene-(1-cyanovinylene)](so-called MEH-CN-PPV), and so on. As those having a luminescent colorof red, for example, poly[2,5-bis(hexyloxy)-1,4-phenylene(lcyanovinylene)] (so-called CN-PPV) orpolythiofine, and so on. As those having a luminescent color of green,for example, poly (para-phenylenevinylene) (so-called PPV) or poly[2-(dimethyloctylsirile)-1,4-phenylenevinylene] (so-called DMOS-PPV),and so on. As those having a luminescent color from blue to green, forexample, m-LPPP, and so on. As those having a luminescent color of blue,for example, poly (paraphenylene) L (so-called PPP), DO-PPP, PDAF, orP3V/PV5, and so on.

Next, the respective structural members contained in the pixel 11 andthe illuminative operation of the pixel 11 are explained with referenceto the equivalent circuit shown in FIG. 3.

As shown in FIG. 3, the gate electrode G of the TFT 13 is connected tothe scanning line 7, the source electrode S is connected to the dataline 6, the drain electrode D is connected to one end of thelight-emitting polymer 14 and the accumulative capacity 15,respectively. The other end of the light-emitting polymer 14 and theaccumulative capacity 15 are commonly connected to a prescribed fixedpotential (not shown), respectively.

During the initial state where the light-emitting polymer 14 is OFF, ascanning signal is not applied to the scanning line 7. Thus , the TFT 13is in an OFF state.

Foremost, pursuant to the data line drive circuit 3 described later, ananalog image signal corresponding to the image signal is supplied to thedata line 6. Then, in the timing corresponding to the supply of suchanalog image signal, a scanning signal is applied to the scanning line 7from the scanning line drive circuit 5, and the TFT 13 becomes an ONstate. As a result, the analog data signal transmitted from the dataline 6 flows from the source electrode S to the drain electrode D, andis further applied to one electrode of the light-emitting polymer 14 andthe accumulative capacity 15.

Thereby, the light-emitting polymer 14 begins to self-illuminate withthe luminance in proportion to the current quantity of the appliedanalog data signal. Simultaneously, electric charge begins accumulatingon the accumulative capacity 15.

Thereafter, even if the supply of the analog data signal from the dataline 6 is finished, the current continues to flow to the light-emittingpolymer 14 and illumination is maintained while the accumulated electriccharge remains in the accumulative capacity 15.

Next, the structure and operation of the data line drive circuit 3 areexplained with reference to FIGS. 4 and 5. FIG. 4 is a block diagramshowing the schematic structure of the data line drive circuit 3. FIG. 5is a circuit diagram showing the detailed structure (of a portioncorresponding to only a single pixel 11) of a second latch circuit andthe D/A converter described later, among the data line drive circuit 3shown in FIG. 4.

Here, the structure of the data line drive circuit 3 explained below isa case wherein the image signal input externally via the mountingterminals 9 is a 3-bit digital image signal. Further, the data linedrive circuit 3 shown in FIG. 4 is of a structure which drives therespective TFTs 13 in a so-called line sequence.

As shown in FIG. 4, the data line drive circuit 3 is structured bycomprising a shift register 20, switches 24 and 25, first latch circuit21, second latch circuit 22, and a D/A converter 23 provided for eachdata line 6.

The first latch circuit 21 is structured of a latch circuit 21A, latchcircuit 21B, and latch circuit 21C in correspondence with the respectivebits in the image signal.

The second latch circuit 22 is structured of a latch circuit 22A, latchcircuit 22B, and latch circuit 22C in correspondence with the respectivebits in the image signal.

Next, the operation is explained.

The switch 25 and the first latch circuit 21 sample the externally input3-bit image signal Sg pursuant to the control of the shift register 20.

Then, in the timing shown with the externally input latch signal S1, theswitch 24 transfers to the respective latch circuits 22A˜22C in thesecond latch circuit 22 digital image signal Sg per each sampled bit.

Thereafter, in the timing of line-sequencing the light-emitting polymer14 in the respective pixels 11, the second latch circuit 22 outputs tothe D/A converter 23 for the respective data lines 6 the transferreddigital image signal Sg per each bit.

Next, the respective D/A converters 23 convert the input digital imagesignal Sg into an analog image signal, and outputs such signal to therespective data lines 6. The analog image signal has a large currentvalue in proportion to the digital value shown with digital image signalSg for each data line 6.

Then, pursuant to the analog image signal, a prescribed current isapplied to the light-emitting polymer 14 via the respective TFTs 13, andthe light-emitting polymer 14 will illuminate.

The detailed structure and operation of the D/A converter 23 are nowexplained with reference to FIG. 5.

As shown in FIG. 5, the D/A converter 23 is structured of switches 30A,31A and TFT 32A provided in correspondence with the first bit signal Sg,showing the first bit (corresponds to 2⁰) in digital image signal Sg;switches 30B, 31B and TFT 32B provided in correspondence with the secondbit signal Sg_(b) showing the second bit (corresponds to 2¹) in digitalimage signal Sg; switches 30C, 31C and TFT 32C provided incorrespondence with the third bit signal Sg_(c) showing the third bit(corresponds to 2²) in digital image signal Sg; TFT 33 provided commonlyto the respective bits and TFT 34 as the additional-basic-currentapplication means, resistors 35˜38, and gate switching circuit 39. Here,as obvious from FIG. 5, the current mirror circuit is structured fromthe respective TFTs 32A, 32B, 32C, 34, and TFT 33.

The channel widths in the respective TFTs 32A, 32B, and 32C have thefollowing relationship. In other words, if the channel width of TFT 32Ais W, the channel width of TFT 32B is 2W, and the channel width of TFT32C is 4W. Here, the channel lengths of TFTs 32A, 32B, 32C, 33, and 34are equal.

According to this structure, the current I which flows to TFT 32A whenboth TFT 33 and TFT 32A simultaneously become the ON state is, when thecurrent flowing to TFT 33 is i and the channel width of TFT 33 is w,

I=i×(2W/w)

Next, the current I′ which flows to TFT 32B when both TFT 33 and TFT 32Bsimultaneously become the ON state is,

I′=i×(2W/w)=2I

Next, the current I″ which flows to TFT 32C when both TFT 33 and TFT 32Csimultaneously become the ON state is,

I″=i×(4W/w)=4I

Meanwhile, the channel width of TFT 34 is the channel width of the flowof the current having current quantity It when both TFT 33 and TFT 34simultaneously become the ON state. The aforementioned current quantityIt is the minimum current quantity within the range of currents in whichthe luminance of the light-emitting polymer changes in proportion to thecurrent quantity pursuant to the current-luminance property of thelight-emitting polymer 14 (cf. FIG. 6).

The operation is now explained.

As shown in FIG. 5, based on the first bit signal Sg_(a) and in thetiming of driving the pixel 11 in line sequence, the latch circuit 22Aturns the switch 31A ON and simultaneously turns the switch 30A OFF whenthe first bit signal Sg_(a) is “1”. Moreover, in the same timing, whenthe first bit signal Sg_(a) is “0”, the latch circuit 22A turns theswitch 31A OFF and simultaneously turns the switch 30A ON.

Similarly , based on the second bit signal Sg_(b) and in the timing ofdriving the pixel 11 in line sequence, the latch circuit 22B turns theswitch 31B ON and simultaneously turns the switch 30B OFF when thesecond bit signal Sg_(b) is “1”. Moreover, in the same timing, when thesecond bit signal Sg_(b) is “0”, the latch circuit 22B turns the switch31B OFF and simultaneously turns the switch 30B ON.

Furthermore, based on the third bit signal Sg_(c) and in the timing ofdriving the pixel 11 in line sequence as with the latch circuit 22A or22B, the latch circuit 22C turns the switch 31C ON and simultaneouslyturns the switch 30C OFF when the third bit signal Sg_(c) is “1”.Moreover, in the same timing, when the third bit signal Sg_(c) is “0”,the latch circuit 22C turns the switch 31C OFF and simultaneously turnsthe switch 30C ON.

TFTs 32A, 32B and 32C respectively form the current mirror circuitbetween TFT 33 based on the operations of the respective switches30A˜30C and 31A˜31C. That is, when the respective bits are “1”, theaforementioned current I, I′, or I″ is supplied to the data line 6, andwhen the respective bits are “0”, no current is supplied.

Currents I, I′, I″ which flowed through TFT 32A, 32B, or 32C aremutually added, and are applied to the drain electrode D of the TFT 13via the data line 6 as analog image signal Sa.

Next, the aforementioned operation is explained by detailed illustrationwith reference to FIG. 5.

In the following explanation, taken as an example is the case where thesecond bit signal Sg_(b) and the third bit signal Sg_(c) arerespectively “1”, and the first bit signal Sg_(a) is “0”; in otherwords, when “6” (=2⁰×0+2¹×1+2²×1) is input as digital image signal Sg.

Digital image signal Sg having the digital value of “6”, is, afterhaving been sampled by the first latch circuit 21 and the switch 25,respectively input to the latch circuits 22A, 22B, and 22C respectivelyas the first bit signal Sg_(a), second bit signal Sg_(b), and third bitsignal Sg_(c).

Here, as the first bit signal Sg_(a) is “0”, the latch circuit 22A turnsthe switch 31A OFF and simultaneously turns the switch 30A ON in thetiming of driving the pixel 11 in a line sequence. Thereby, current Idoes not flow in TFT 32A.

Meanwhile, as the second bit signal Sg_(b) is “1”, the latch circuit 22Bturns the switch 31B OFF and simultaneously turns the switch 30B ON inthe timing of driving the pixel 11 in a line sequence. Thereby, currentI″ (=2I) flows in TFT 32B.

Moreover, as the third bit signal Sg_(c) is “1”, the latch circuit 22Cturns the switch 31C OFF and simultaneously turns the switch 30C ON inthe timing of driving the pixel 11 in a line sequence. Thereby, currentI″ (=4I) flows in TFT 32C.

Accordingly, the current value supplied to the TFT 13 as the analogimage signal is,

2I+4I=6I

In other words, in comparison to the digital value “6” input as digitalimage signal Sg, the current value supplied as analog image signal Sa is6I. Thus, the light-emitting polymer 14 illuminates in the luminancecorresponding to the digital value “6” (i.e.; six times the luminance ofthe luminance corresponding to the digital value “1”).

Contrarily, in parallel to the operation of TFTs 32A˜32C, the gateswitching circuit 39 turns the TFT 34 ON when one of the signal a songthe first bit signal Sg_(a)˜third bit signal Sg_(c) is “1”.

Here, TFT 34 continuously forms the current mirror circuit between TFT33. When TFT 34 is turned ON, supplied to the data line 6 as theadditional basic current is a current having the smallest currentquantity It among the range of currents in which the luminance of thelight-emitting polymer 14 changes in proportion to the current quantity.

As a result, when the light-emitting polymer 14 in the pixel 11 Is to belit in a certain luminance, the additional basic current having currentvalue It is constantly superimposed and flowed to analog image signalSa.

Therefore, as analog image signal Sa is applied within the range wherethe luminance of the light-emitting polymer 14 changes in proportion tothe current value, the light-emitting polymer 14 illuminates in aluminance accurately in proportion to the current value of analog imagesignal Sa (i.e.; digital value of digital image signal Sg).

As described above, according to the operation of the display device 1of the present embodiment, the current-driven light-emitting polymer 14is driven by the current-adding D/A converter 23 and realizes a drivewith large driving power. Moreover, since currents that only directlydrive the light-emitting polymer 14 are used, electric power consumptionis lowered by the generation of wasteful drive currents beingsuppressed.

Furthermore, as the light-emitting polymer 14 is driven upon providing aTFT 13 to each pixel 11, it is possible to display a high-quality imagehaving high resolution and without any crosstalk in the picture.

In addition, as the respective TFTs 13 are thin film transistors formedfrom polysilicon, even if a strong current for driving thelight-emitting polymer 14 is flowed for a long period of time, thedriving power thereof will not lower.

Moreover, as analog image signal Sa is applied upon structuring acurrent mirror circuit in the D/A converter 23, it is possible toefficiently supply analog image signal Sa to the light-emitting polymer14.

Further, in comparison with other D/A converter systems, as the numberof elements required for the structure is extremely small, this isparticularly adequate for drive circuits required to be arranged in anarrow pitch as in display devices.

As the element which self illuminates is a light-emitting polymer 14,obtained are images of high intensity and abundant color reproducibilityby molecular-designing appropriate organic materials.

Although the aforementioned embodiment was an explanation employing thelight-emitting polymer 14 as the light-emitting element, the presentinvention may otherwise be widely employed to display elements usingcurrent-driven light-emitting elements such as organic or inorganic EL(Electro Luminescence) elements and the like.

(II) Embodiments of the Electronic Apparatus

Next, embodiments of various electronic apparatus employing the displaydevice 1 of the aforementioned embodiment are explained with referenceto FIGS. 7˜9.

The electronic apparatus structured upon employing the display device 1above, as shown in FIG. 7, includes a display information output source1000, display information processing circuit 1002, display drive circuit1004, display panel 1006, clock generation circuit 1008, and powersource circuit 1010.

Among the above, the display information output source 1000 includes amemory such as a ROM (Read Only Memory), RAM (Random Access Memory),tuning circuit for tuning and outputting a television signal, and so on.The display information output source 1000 outputs display informationsuch as a video signal based on the clock signal from the clockgeneration circuit 1008.

The display information processing circuit 1002 processes and outputsthe display information based on the clock signal from the clockgeneration circuit 1008. This display information processing circuit1002 may include, for example, an amplification circuit, mutualdevelopment circuit, rotation circuit or clamping circuit, etc.

The display drive circuit 1004 is structured by including a scanningside drive circuit and data side driving circuit. The display drivecircuit 1004 drives the display panel 1006 for display.

The power source circuit 1010 supplies electricity to the respectivecircuits mentioned above.

As the electronic apparatus having the aforementioned structure, listedmay be a personal computer (PC) and engineering workstation (EWS) shownin FIG. 8 in compliance with multimedia, or a cellular phone, wordprocessor, television, view-finder style or direct monitor-viewing typevideo tape recorder, electronic notebook, electronic desk-topcalculator, car-navigation device, POS terminal, device comprising atouch panel, and so on.

The personal computer shown in FIG. 8 has a main body comprising akeyboard 1202, and a display 1206 including the display device of thepresent invention.

What is claimed is:
 1. A display device comprising: a plurality oflight-emitting elements; and a D/A converter for providing an analogsignal to the plurality of light-emitting elements, the D/A converterincluding a first transistor and a plurality of transistors, the firsttransistor enabling to constitute a current mirror circuit with each ofthe plurality of transistors, each of the plurality of transistorshaving a gate controlled by a respective bit signal of a digital signalto obtain a current for a current flow through the respective transistoraccording to a binary state of the respective bit signal, the D/Aconverter converting the digital signal to an analog signal by adding upthe current flows through the plurality of transistors, and the analogsignal having a current value according to the digital signal.
 2. Thedisplay device according to claim 1, the plurality of said transistorshaving different channel widths from each other.
 3. The display deviceaccording to claim 1, the analog signal being generated by adding anadditional basic current flowing through the plurality of saidtransistors.
 4. The display device according to claim 3, furthercomprising a second transistor through which the additional basiccurrent flows.
 5. The display device according to claim 3, theadditional basic current having current level being less than minimumcurrent level within a range of current level in proportion to luminancelevel of a plurality of said light-emitting elements.
 6. The displaydevice according to claim 1, a plurality of said light-emitting elementshaving a light-emitting polymer.
 7. An electric apparatus comprisingsaid display device according to claim
 1. 8. A display devicecomprising: a data line driving circuit including a plurality oftransistors, each of the plurality of transistors having a gatecontrolled by a respective bit signal of a digital signal to obtain acurrent for a current flow through the respective transistor accordingto a binary state of the respective bit signal, said data line drivingcircuit converting the digital signal to an analog signal correspondingto the digital signal by adding up the currents flowing through theplurality of said transistors, the analog signal having a current valueaccording to the digital signal; a plurality of scanning lines to supplya scanning signal; a plurality of data lines connected to said data linedriving circuit to supply the analog signal; a plurality of pixels, eachof said pixels comprising: a switching element connected to one of aplurality of said scanning lines and one of a plurality of said datalines; and a light-emitting element emitting in accordance with theanalog signal; and a current mirror circuit constituted between each ofthe plurality of transistors established corresponding to digitalsignals and a first transistor.
 9. The display device according to claim8, the switching element being a polysilicon thin-film transistor. 10.The display device according to claim 8, the analog signal beinggenerated by adding an additional basic current flowing through theplurality of said transistors.
 11. The display device according to claim10, further comprising a second transistor through which the additionalbasic current flows.
 12. The display device according to claim 10, theadditional basic current having current level being less than minimumcurrent level within a range of current level in proportion to luminancelevel of a plurality of said light-emitting elements.
 13. The displaydevice according to claim 8, a plurality of said light-emitting elementshaving a light-emitting polymer.
 14. An electric apparatus comprisingsaid display device according to claim
 8. 15. A circuit for converting adigital signal to an analog signal, the circuit comprising a pluralityof transistors, each of the plurality of transistors having a gatecontrolled by a respective bit signal of a digital signal to obtain acurrent for a current flow through the respective transistor accordingto a binary state of the respective bit signal, the analog signalgenerated by adding up the currents flowing through the plurality oftransistors, and the analog signal having a current value according tothe digital signal; and a current mirror circuit constituted betweeneach of the plurality of transistors established corresponding todigital signals and a first transistor.
 16. A circuit for converting adigital signal to an analog signal, the circuit comprising a pluralityof transistors, each of the plurality of transistors having a gatecontrolled by a respective bit signal of a digital signal to obtain acurrent for a current flow through the respective transistor accordingto a binary state of the respective bit signal, the analog signalgenerated by adding an additional current to the currents flowingthrough the plurality of transistors, and the analog signal having acurrent value according to the digital signal; and a current mirrorcircuit constituted between each of the plurality of transistorsestablished corresponding to digital signals and a first transistor. 17.A circuit for converting a digital signal to an analog signal, thecircuit comprising: a plurality of transistors, each of the plurality oftransistors having a gate controlled by a respective bit signal of adigital signal to obtain a current for a current flow through therespective transistor according to a binary state of the respective bitsignal; and a first transistor enabling to constitute a current mirrorcircuit with each of the plurality of transistors, wherein an analogsignal having a current value according to the digital signal isgenerated.
 18. A circuit for converting a digital signal to an analogsignal, the circuit comprising a plurality of transistors, each of theplurality of transistors having a gate controlled by a respective bitsignal of a digital signal to obtain a current for a current flowthrough the respective transistor according to a binary state of therespective bit signal, the plurality of transistors having differentchannel widths, the analog signal generated by adding an additionalcurrent to the currents flowing through the plurality of transistors,and the analog signal having a current value according to the digitalsignal; and a current mirror circuit constituted between each of theplurality of transistors established corresponding to digital signalsand a first transistor.